The purpose of this research study is to determine how Lubiprostone, a medication used to treat irritable bowel syndrome with constipation predominant symptoms (IBS-C), works to reduce clinical pain. Lubiprostone acts in the small intestine to cause an increase in the secretion of chloride, water and sodium. The increased fluid causes food residue to move through the bowel more quickly and makes the stools softer. First, we want to test the idea that Lubiprostone works by making a person less sensitive to pain. Second, we want to confirm that Lubiprostone decreases the time it takes fecal matter to travel through your GI tract, referred to as transit time.

Subjects will be enrolled in an 8-week study requiring a total of 7 visits to the UNC Center for Clinical and Translational Research. The protocol is divided into 4 two-week periods: (1) Two-week baseline diary symptom monitoring, followed by a barostat test of pain sensitivity. (2) Two-week treatment with either Lubiprostone or placebo, with daily symptom diary recording and barostat test of pain sensitivity at the end. In addition, patients will be tested for whole gut transit time by the radio-opaque marker (Sitzmark) technique in the second week. (3) Two-week washout period, during which patients will continue the symptom diary. (4) Two-week crossover to Lubiprostone or placebo, identical to the second two-week period.

Inclusion Criteria:

• clinical diagnosis of IBS-C
• meeting Rome III diagnostic criteria for IBS-C
• age 18 or older

Exclusion Criteria:

• use of laxatives or prokinetics within two weeks prior to the study or during the study
• use of IBS-specific compounds, opiates, anticholinergics, or any drug likely to cause constipation as a side-effect
• use of analgesics for 48 hours prior to the study
• hypothyroid condition
• history of bowel resection except appendectomy or cholecystectomy
• psychotic disorder, major depression, substance abuse (other than tobacco), or other psychiatric condition likely to interfere with the conduct of the study. Subjects treated for depression more than 2 years ago or for situational circumstances may be eligible for the study at the investigator's discretion
• renal disease
• inflammatory or ischemic disease of the rectum
• known to be an unreliable subject
• Because this study involves exposure to radiation, subjects who are pregnant or planning to become pregnant, employees currently working with radiation, and subjects who have participated in research involving radiation within the past year will also be excluded.